1. Field of the Invention
This invention relates to a way of sending two-level information, such as binary digital data over long distances in such a way that the overall system will support almost any signal format, so long as the bandwidth of the system is not exceeded and as long as the format is of the two-level class. The effectiveness of the present invention is based on the novel use of a wide acceptance band at the receiving portion of each level-crossing remodulation function that is spaced at intervals along the transmission path, combined with the fact that at the final receiver, fairly large amounts of timing jitter can accumulate without seriously affecting the error rate, because the accumulated jitter in level crossing instant is only weakly converted into amplitude fluctuations at the point in the final receiver at which a decision is made between levels.
2. Discussion of the Prior Art
Repeaters and regenerators have been used for decades to allow transmission of electrical signals over links of great distances. Such units are interposed at spaced points on the link in such a way as to restore the attenuated and thus noisy signal back to full strength at each repeater or regenerator. Repeaters recreate the entire waveform, amplitude and phase alike, while regenerators, which are used with digital transmissions, make bit-by-bit decisions that are rendered more accurate by recovering bit timing by a process of smoothing over many bit times (often thousands), typically using a phase-locked loop with a long integration time. Repeaters are simply amplifiers, and since each must recreate the full waveform, are very vulnerable to noise. Regenerators are designed to operate at a single bit rate, and therefore the receiver, which includes the phase-locked loop, must have an acceptance bandwidth wide enough to pass the bit rate. Because of this narrow acceptance band, regenerators are much more effective against noise than repeaters and are therefore widely used for digital transmission. However, they require clocking at a known bit rate at each regenerator. The ability to build longer links with digital regenerators than is possible with analog repeaters is one of the reasons that digital transmission has almost completely replaced analog transmission in the world's telephone systems. Both repeaters and regenerators are well described in the article by E. O. Sunde, "Self-Timing Regenerative Repeaters," in the Bell System Technical Journal, vol. 36, July, 1957, and in Chapters 1 and 2 of the book by P. Trischitta, Jitter in Digital Transmission Systems, Artech House, 1989.
In modern information systems, such as computer networks the problem of transmitting a variety of signal formats over long distances arises very commonly, and flexibility of use often demands a capability to send one format for some time period and then another with a different format over the same path. A different format usually means a different bit rate. Repeaters will provide the bit rate insensitivity desired, but are not a good solution for long distances, since the noise accumulates too rapidly along the sequence of repeaters. Regenerators are not a satisfactory solution either, if bit-rate independence is a requirement, because they involve a commitment to one chosen bit rate; moreover, they do not function with FM, where the level crossings, instead of being at regular intervals as with data, vary widely in their spacing in time.
Thus, repeaters have the desired format independence but poor noise performance, whereas with regenerators the converse is true. What is needed is a way of building long transmission links by inserting at appropriate points on the overall link some kind of device such that by the time the signal is observed at the final receiver, the noise will not have built up as rapidly as it would with a series of repeaters, and yet the devices must possess the bit rate independence that is not available with regenerators.
The present invention satisfies this requirement. It is based on simply passing on the level-crossing times at each stage, and adding a wide bandwidth capability to each receiver filter preceding the level-crossing determination at each stage, and also capitalizing on the surprisingly small conversion of timing jitter into amplitude noise at the final receiver when clock recovery by long smoothing times is used, as with a phase-locked loop. The process is called here "level-crossing remodulation". The fact that the conversion of phase jitter into amplitude fluctuations is weak has been well documented in the literature, for example, in FIGS. 4.5 and 4.6 of the book by P. Trischitta, without an analysis having been made up to now of how this weak dependence can be exploited in a series of remodulators that only retain level crossing time information. The idea of passing on only the level crossing timing information while throwing away all amplitude information is mentioned in passing and rejected as unworkable in Section 1.1 of the paper by E. O. Sunde, "Self-Timing Regenerative Repeaters," in the Bell System Technical Journal, vol. 36, July, 1957, but only in the context of a single predetermined bit rate. In addition, Sunde does not mention the use of a wideband filter and does not address the general problem of designing a long link that will handle arbitrary bit rates. The idea of building long links by repeated recreation of the level crossing instants, where at each such remodulation the receiver bandwidth is maintained wide enough to allow bit rate independence and of exploiting the weak conversion of timing jitter into amplitude noise at the final receiver are not known in the literature.